The invention relates to the treatment of cancer and other proliferative diseases.
One way by which cells die is referred to as apoptosis, or programmed cell death. Apoptosis often occurs as a normal part of the development and maintenance of healthy tissues. The process may occur so rapidly that it is difficult to detect.
The apoptosis pathway is now known to play a critical role in embryonic development, viral pathogenesis, cancer, autoimmune disorders, and neurodegenerative diseases, as well as other events. The failure of an apoptotic response has been implicated in the development of cancer, autoimmune disorders, such as lupus erythematosis and multiple sclerosis, and in viral infections, including those associated with herpes virus, poxvirus, and adenovirus.
The importance of apoptosis in cancer has become clear in recent years. The identification of growth promoting oncogenes in the late 1970's gave rise to an almost universal focus on cellular proliferation that dominated research in cancer biology for many years. Long-standing dogma held that anti-cancer therapies preferentially targeted rapidly dividing cancer cells relative to “normal” cells. This explanation was not entirely satisfactory, since some slow growing tumors are easily treated, while many rapidly dividing tumor types are extremely resistant to anti-cancer therapies. Progress in the cancer field has now led to a new paradigm in cancer biology wherein neoplasia is viewed as a failure to execute normal pathways of programmed cell death. Normal cells receive continuous feedback from their neighbors through various growth factors, and commit “suicide” if removed from this context. Cancer cells somehow bypass these commands and continue inappropriate proliferation. It is now believed that many cancer therapies, including radiation and many chemotherapy regimens, previously thought to act by causing cellular injury, actually work by triggering apoptosis.
Both normal cell types and cancer cell types display a wide range of susceptibility to apoptotic triggers, although the determinants of this resistance are only now under investigation. Many normal cell types undergo temporary growth arrest in response to a sub-lethal dose of radiation or cytotoxic chemical, while cancer cells in the vicinity undergo apoptosis. This differential effect at a given dose provides the crucial treatment window that allows successful anti-cancer therapy. It is therefore not surprising that resistance of tumor cells to apoptosis is emerging as a major basis for failure of cancer treatments.
Several potent endogenous proteins that inhibit apoptosis have been identified, including the Bcl-2, and IAP protein families in mammals. Certain members of the IAP family directly inhibit terminal effector caspases, i.e., casp-3 and casp-7, engaged in the execution of cell death, as well as the key mitochondrial initiator caspase, casp-9, important to the mediation of cancer chemotherapy induced cell death. The IAPs are the only known endogenous caspase inhibitors, and thus play a central role in the regulation of apoptosis.
The IAPs have been postulated to contribute to the development of some cancers, and a postulated causal chromosomal translocation involving one particular IAP (cIAP2/HIAP1) has been identified in MALT lymphoma. A recent correlation between elevated XIAP, poor prognosis, and short survival has been demonstrated in patients with acute myelogenous leukemia. Furthermore, XIAP was highly over-expressed in many tumor cell lines of the NCI panel.
There exists a need for improved cancer therapeutics and, in particular, therapeutics that can induce cancer cells to undergo apoptosis and override anti-apoptotic signals provided in such cells.